1. Field of the Invention
The present invention relates to an adjustment method such as a color registration adjustment for an electrophotographic type image forming apparatus, and an image forming apparatus, and more specifically relates to an image adjustment method and an image forming apparatus which are capable of automatically adjusting misregistration of a multi-color image which occurs when forming the multi-color image by superposing color component images formed on an image carrier (image forming means: photoconductor drum) or a transfer carrier (transfer medium: transfer belt, paper).
2. Description of Related Art
In an image forming apparatus such as a digital color copying machine, after decomposing inputted data into a plurality of color components and performing image processing, the respective color component images are superposed to form a multi-color image. When the images formed by the respective color components are not accurately superposed during the forming a multi-color image, misregistration occurs in the resultant multi-color image, and the image quality deteriorates.
In particular, in an image forming apparatus comprising image forming units, each being used exclusively by each color component, to improve the forming speed of a multi-color image, images are formed by respective color components in the respective image forming units, and then the images formed by the respective color components are superposed one upon another to form a multi-color image. In such an image forming apparatus, there tend to be differences in the transfer positions of the images formed by the respective color components, and consequently there arises a serious problem of misregistration of the multi-color image.
Therefore, in order to accurately superpose the images formed by the respective color components, an image forming apparatus is designed to perform a color registration adjustment to adjust the misregistration of a multi-color image, and thereby form a satisfactory multi-color image having no misregistration.
The color registration adjustment is usually carried out by using an optical detector to detect a displacement of the image forming position of other color component with respect to the image forming position of a color component to be the reference. Based on the result of the detection, an adjusting amount is determined. Then, according to the adjusting amount, the timing of forming an image by each color component is adjusted so that the transfer positions of the images formed by the respective color component images coincide with each other. In general, the images formed by the respective color components are transferred at the same timing, and the distance between the transfer positions of the images formed by the respective color components is detected, or the density of a multi-color image formed by superposing the images of respective color components is detected.
For example, in an image forming apparatus disclosed in Japanese Patent Application Laid-Open No. 10-213940 (1998), the distance between the transfer positions of the images formed by the respective color components is detected, and an adjustment is made based on the detected amount of displacement between the transfer positions. Specifically, the distance between an image formed by a color component to be the reference and an image formed by other color component is detected with a detector, and the amount of displacement between the transfer positions of the images formed by the respective color components is determined based on the detected distance, thereby adjusting the misregistration.
Further, Japanese Patent Application Laid-Open No. 2000-81744 discloses an image forming apparatus which measures the density of a multi-color image formed by superposing images formed by respective color components, and adjusts misregistration so that the measured density becomes equal to a density of a state in which the images formed by the respective color components are accurately superposed.
In this image forming apparatus, in order to improve the adjustment accuracy, an image to be formed by each color component is formed by repeatedly forming a plurality of same images. A plurality of line images are formed as the same images, and the density of a multi-color line image is detected with a detector to find the superposed state of the line images formed by the respective color components. Then, a state in which the density of the multi-color line image detected with the detector is within a predetermined density range is regarded as a state in which the line images formed by the respective color components are accurately superposed, and an adjustment is made so that image forming is performed in this superposed state, thereby performing the color registration adjustment.
In the image forming apparatus of Japanese Patent Application Laid-Open No. 10-213940 (1998), since the displacement between the transfer positions of the respective images is found using the detector for detecting the transfer positions of the images formed by the respective color components, there is a problem that a detector with high detection accuracy needs to be used to detect a very small displacement between the transfer positions.
On the other hand, in the image forming apparatus disclosed in Japanese Patent Application Laid-Open No. 2000-81744, since an adjustment value for a state in which the reference image and the image formed by a color component to be adjusted perfectly overlap is found by detecting densities while shifting the adjustment value on a line by line basis over the entire image color registration adjustment range, this apparatus has the advantage that there is no need to use a detector with high detection accuracy.
However, both of these image forming apparatuses are susceptible to the following problems: an erroneous detection result may be obtained due to a surface condition (particularly, scratches) of a transfer unit where images are formed; an error may occur during the execution of color registration adjustment, and the quality of the formed image may rather deteriorate after the color registration adjustment. These problems appear particularly in a type of apparatus in which the detector detects specular reflected light. These problems are also described in Japanese Patent Application Laid-Open No. 2001-312116.